Hot-Rolled Flat Steel Product and Method for the Production Thereof

ABSTRACT

A hot-rolled flat steel product having a thickness of &lt;1.5 and consisting of, in % by mass, C: 0.04-0.23%, Si: 0.04 0.54%, Mn: 1.4-2.9%, Ti+V, wherein 0.005%&lt;%Ti+%V&lt;0.15%, and optionally one or more elements of Al, Cr, Mo, and B, where AI: 0.01-1.5%, 0.02&lt;%Mo+%Cr&lt;1.4%, and B: 0.0005-0.005%, and the remainder consisting of iron and inevitable impurities. The structure of the flat steel product consists of, in percent by area, in sum, 50-90% ferrite and bainite ferrite, 5-50% martensite, 2-15% residual austenite and &lt;10% other structure elements. The flat steel product has a yield point Rp0.2&gt;290 MPa, a tensile strength Rm&gt;490 MPa and an elongation at break A80 where A80[%]=B−Rm/37 with 31&lt;B&lt;51. To at least one surface of the flat steel product, a Zn coating is applied by hot-dip coating. Also a method for producing a flat steel product of this kind.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the United States national phase of InternationalApplication No. PCT/EP2020/061200 filed Apr. 22, 2020, the disclosure ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a hot-rolled flat steel product comprising asteel substrate and a corrosion protection layer applied thereto byhot-dip coating on zinc.

In addition, the invention relates to a method for producing such a flatsteel product.

Description of Related Art

Flat steel products are understood in the present text as rolledproducts of which the length and width are each significantly greaterthan their thickness. These include in particular steel strips and steelsheets.

In the present text, unless explicitly noted otherwise, informationabout the contents of alloying constituents is always made in % by mass.

The proportions of certain constituents on the structure of the steelsubstrate of a flat steel product are given in % by area, unlessotherwise noted.

In the present text, the term “impurities” of a steel, zinc or otheralloy refers to technically unavoidable materials accompanying thesteel, which can enter the steel during production or cannot be removedcompletely therefrom, but the contents of which are in any case so smallthat they have no influence on the properties of the steel.

The image analysis for the quantitative determination of structure takesplace optically by means of light optical microscopy (“LOM”) with 200 to2000 times and with a scanning electron microscope (“SEM”) with 2000 to20,000 times magnification.

The distribution of manganese (Mn) in the structure of the steelsubstrate of a flat steel product according to the invention has beendetermined by wavelength-dispersive X-ray microrange analysis (WDX) ofthe structure, which has been described, for example, by Reimer L.(1998) in “Elemental Analysis and Imaging with X-Rays” appearing inScanning Electron Microscopy, Springer Series in Optical Sciences, vol.45, Springer, Berlin, Heidelberg.

The strength and expansion properties mentioned here, such as tensilestrength Rm, yield point Rp0.2, uniform elongation Ag, elongation A50and elongation A80 of flat steel products were determined in the tensiletest according to DIN-EN 6892-1:2017, unless stated otherwise.

High-load passenger car and truck components, such as crash structuresand chassis of automobile bodies, require a hot-dip galvanized steelsheet with a thickness of more than 1.5 mm and a tensile strength ofmore than 590 MPa.

Frequently, hot-rolled flat steel products that consist of complex phasesteels (CP-W), the structure of which consists largely of bainite, areused for the production of such components. However, CP-W steels sufferfrom relatively lower deformability, which prevents the design ofgeometrically complex components.

Dual-phase steels (DP), which consist of a combination of hard (e.g.,martensite or bainite) and soft (e.g., ferrite) phases, are suitable forcomplex components due to their combination of high strength and gooddeformability. However, cold-rolled dual-phase steels (DP-K) withthicknesses greater than 1.5 mm have a higher sensitivity to surfacedefects, such as ungalvanized areas. Therefore, the maximum sheetthickness of hot-dip galvanized DP-K steels is generally limited to 2mm.

The direct galvanizing of hot-rolled dual-phase steels (DP-W) islikewise not feasible. For galvanizing, the sheet must be heated totemperatures of greater than 460° C. (the zinc bath temperature).However, at these temperatures, the hard component of the structure,particularly martensite, is tempered and the DP characteristics arelost.

One possibility would be to perform annealing and then galvanizing of ahot-rolled strip in a hot-dip galvanizing line with an annealing cycletypical for DP-K (i.e., partial austenitization in the intercriticaltemperature range, i.e., in the temperature range lying between the Ac1and Ac3 temperatures of the respective steel, where α- and γ-Fe areproduced in equilibrium). This procedure is similar to the manufacturingprocess of a DP-K steel except for the cold-rolling step. However, thereis the risk here of the omission of the cold-rolling step leading topoorer mechanical properties compared to those of a DP-K steel.

High-strength multi-phase steel with minimum tensile strengths of 580MPa is known from DE 10 2012 013 113 A1. The steel should preferablyhave a dual-phase structure and make it possible to produce cold-rolledor hot-rolled steel strips with improved forming properties, from which,in particular, parts for lightweight vehicle construction can beproduced. For this purpose, the known multi-phase steel consists of, in% by mass, 0.075%≤C≤0.105%, 0.600%≤Si≤0.800%, 1.000%≤Mn≤2.250%,0.280%≤Cr≤0.480%, 0.010%≤Al≤0.060%, ≤0.020% P, ≤0.0100% N, ≤0.0150% Sand the remainder consisting of iron and impurities.

A further high-strength multi-phase steel with a minimum tensilestrength of 580 MPa is the steel known from DE 10 2012 006 017 A1. Thisshould also preferably have a dual-phase structure and be suitable forthe production of cold-rolled or hot-rolled steel strips, which havegood forming properties. As such, parts for lightweight vehicleconstruction are to be formed from such steel strips in particular. Forthis purpose, the known steel consists of, in % by mass,0.075%≤C≤0.105%, 0.200%≤Si≤0.300%, 1.000%≤Mn≤2.000%, 0.280%≤Cr≤0.480%,0.010%≤Al≤0.060%, up to 0.020% P, 0.005%≤Nb≤0.025%, up to 0.0100% N, upto 0.0050% S, and the remainder consisting of iron and technicallyunavoidable impurities.

The steel known from DE 10 2013 013 067 A1 is also among the type ofknown multi-phase steels explained above, which preferably have adual-phase structure and are intended to be suitable for cold-rolled orhot-rolled steel strip with improved forming properties. This knownsteel should have a yield-to-tensile ratio of not more than 73% and, in% by mass, 0.075%≤C≤0.105%, 0.600%≤Si≤0.800%, 1.000%≤Mn≤1.900%,0.100%≤Cr≤0.700%, 0.010%≤Al≤0.060%, 0.0020%≤N≤0.0120%, ≤0.0030% S,0.005%≤Nb≤0.050%, 0.005%≤Ti≤0.050%, 0.0005%≤B≤0.0040%, ≤0.200% Mo,≤0.040% Cu, ≤0.040% Ni and the remainder consisting of iron andunavoidable impurities.

SUMMARY OF THE INVENTION

Against the background of the prior art explained above, the objectarises to develop a flat steel product that not only has optimizedmechanical properties, but is also particularly suitable for applicationof a Zn-based corrosion protection layer by hot-dip coating.

The invention has achieved this object by a flat steel product thatcomprises a steel substrate at least 1.5 mm thick, which consists of, in% by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, whereinthe following applies for the sum %Ti+%V of the contents of Ti and V:0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of theelements from the group “Al, Cr, Mo, B” with the specification thattheir contents, if present, are dimensioned as follows: Al: 0.01-1.5%,Cr and Mo, wherein the following applies for the sum of the contents%Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, B: 0.0005-0.005%, and theremainder consisting of iron and unavoidable impurities, wherein theunavoidable impurities include less than 0.02% P, less than 0.005% S,less than 0.01% N and less than 0.005% Nb. The flat steel product has astructure which consists of, in % by area, in total 50-90% ferrite andbainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to10% other structural constituents that are unavoidable due toproduction, and has a yield point Rp0.2 of at least 290 MPa, a tensilestrength Rm of at least 490 MPa and an elongation at break A80 that isdetermined by the following formula (1): A80[%]=B−Rm/37 with 31≤B≤51. Acorrosion protection layer based on zinc is applied to at least one ofthe surfaces by hot-dip coating

Furthermore, the invention should specify a method with which theproduction of flat steel products obtained according to the invention isreliably achieved.

To achieve this object, the invention has proposed the method specifiedin which at least the following work steps are performed. A hot-rolledsteel substrate in the form of a steel strip in at least the followingsub-steps: A.1) melting a steel melt, which consists of, in % by mass,C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the followingapplies for the sum %Ti+%V of the contents of Ti and V:0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of theelements from the group “Al, Cr, Mo, B” with the specification thattheir contents, if present, are dimensioned as follows: Al: 0.01-1.5%,Cr and Mo, wherein the following applies for the sum of the contents%Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, B: 0.0005-0.005%, and theremainder consisting of iron and unavoidable impurities, wherein theunavoidable impurities include less than 0.02% P, less than 0.005% S,less than 0.01% N and less than 0.005% Nb; A.2) casting the steel meltto form a preliminary product, which is a slab or thin slab; A.3)preheating the preliminary product at a preheating temperature that isat least 1150° C. and at most 1350° C.; A.4) hot-rolling the preliminaryproduct to form a hot-rolled steel strip, wherein the final temperatureof the hot rolling is at least 840-980° C. and the thickness of thehot-rolled steel strip is 1.5-10 mm; A.5) cooling the hot-rolled steelstrip to a coiling temperature that is 510-640° C.; and A.6) coiling thehot-rolled steel strip cooled to the coiling temperature. The steelsubstrate, which is present in the form of a hot-rolled steel strip, iscoated with a corrosion protection coating based on zinc in at least thefollowing sub-steps, which are passed through continuously: B.1)optional pickling of the hot-rolled steel strip; B.2) heating thehot-rolled steel strip with a heating rate of 0.5-100° C./s to anannealing temperature of 750-950° C. and holding the hot-rolled steelstrip at the annealing temperature over an annealing period of 10-1000s;

B.3) cooling the hot-rolled steel strip at a cooling rate of 0.5-100°C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.)applies, wherein the temperature of the zinc melt bath is referred to asBT, which is 450-480° C.; B.4) passing the hot-rolled steel strip cooleddown to the bath entry temperature BET through the zinc melt bath, whichconsists of up to 5% by mass Mg, up to 10% by mass Al, the remainder ofZn and unavoidable impurities; B.5) cooling the obtained flat steelproduct with a cooling rate of 0.5-100° C./s; and B.6) optionalskin-pass rolling of the flat steel product with a degree of skinpassing of 0.3-2.0%. It is self-evident that, when carrying out themethod according to the invention, the person skilled in the art doesnot only carry out the method steps mentioned in the claims andexplained here, but also carries out all other steps and activities thatare regularly carried out in the prior art upon the practicalimplementation of such methods, if the necessity arises for this.

Advantageous embodiments of the invention are are explained in detailbelow, as is the general inventive concept.

DESCRIPTION OF THE INVENTION

The invention thus provides a hot-rolled flat steel product thatcomprises a steel substrate and a corrosion protection layer based onzinc (Zn) applied thereto by hot-dip coating.

Thereby, the steel of the steel substrate of a flat steel productaccording to the invention consists of, in % by mass,

-   C: 0.04-0.23%,-   Si: 0.04-0.54%,-   Mn; 1.4-2.9%,-   Ti+V, wherein the following applies for the sum %Ti+%V of the    contents of Ti and V: 0.005%≤%Ti+%V≤0.15%,    and optionally one element or a plurality of the elements from the    group “Al, Cr, Mo, B” with the specification that their contents, if    present, are dimensioned as follows:-   Al: 0.01-1.5%-   Sum of contents of Cr+Mo: 0.02-1.4%-   B: 0.0005-0.005    and the remainder consisting of iron and unavoidable impurities,    wherein the unavoidable impurities include less than 0.02% P, less    than 0.005% S, less than 0.01% N and less than 0.005% Nb.

Thereby, the steel substrate of a flat steel product according to theinvention is at least 1.5 mm thick and has a structure that consists of,in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50%martensite, 2-15% residual austenite and up to 10% other structuralconstituents that are unavoidable due to production.

At the same time, a flat steel product according to the invention has ayield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least490 MPa and an elongation at break A80 that is determined by thefollowing formula (1):

A80[%]=B−Rm/37 where 31≤B≤51.

A flat steel product according to the invention can be produced bypassing through at least the following work steps:

-   A) Producing a hot-rolled steel substrate in the form of a steel    strip in at least the following sub-steps:-   A.1) Melting of a steel composed according to the specification of    the invention;-   A.2) Casting the steel melt to form a preliminary product, which is    a slab or thin slab;-   A.3) Preheating the preliminary product at a preheating temperature    that is at least 1150° C. and at most 1350° C.;-   A.4) Hot-rolling the preliminary product to form a hot-rolled steel    strip, wherein the final temperature of the hot rolling is at least    840-980° C. and the thickness of the hot-rolled steel strip is    1.5-10 mm;-   A.5) Cooling the hot-rolled steel strip to a coiling temperature    that is 510-640° C.;-   A.6) Coiling the hot-rolled steel strip cooled to the coiling    temperature.-   B) Coating the steel substrate, which is present in the form of a    hot-rolled steel strip, with a corrosion protection coating based on    zinc in at least the following sub-steps, which are passed through    continuously:-   B.1) Optional pickling of the hot-rolled steel strip;-   B.2) Heating the hot-rolled steel strip with a heating rate of    0.5-100° C./s to an annealing temperature of 750-950° C. and holding    the hot-rolled steel strip at the annealing temperature over an    annealing period of 10-1000 s;-   B.3) Cooling the hot-rolled steel strip at a cooling rate of    0.5-100° C./s to a bath entry temperature BET, for which    BT≤BET≤(BT+20° C.) applies, wherein the temperature of the zinc melt    bath is referred to as BT, which is 450-480° C.;-   B.4) Passing the hot-rolled steel strip cooled down to the bath    entry temperature BET through the zinc melt bath, which consists of    up to 5% by mass Mg, up to 10% by mass Al, the remainder of Zn and    unavoidable impurities;-   B.5) Cooling the obtained flat steel product with a cooling rate of    0.5-100° C./s;-   B.6) Optional skin-pass rolling of the flat steel product with a    degree of skin passing of 0.3-2.0%.

A preheating temperature of at least 1150° C. is necessary in work stepA.1 in order to completely homogenize the structure of the preliminaryproduct. At lower temperatures, the microstructure of the preliminaryproduct would be inherited by the hot strip subsequently produced, sothat the Mn segregations desired according to the invention could not beformed. Likewise, at lower preheating temperatures, the alloyingelements would be bound in deposits, so that their effects on themechanical properties of a flat steel product according to the inventioncould not develop.

A hot-rolling end temperature of at least 840° C. is required to be ableto roll the preliminary product alloyed according to the invention in areliable manner to form a hot-rolled steel strip. At lower hot-rollingend temperatures, the rolling forces would be too high and, as a result,the risk of damage to the rolls of the roll stands used for hot rollingwould increase disproportionately. In order to minimize such risks, ahot-rolling end temperature of at least 880° C. can be provided. Thehot-rolling end temperature should not exceed 980° C., since hot-rollingend temperatures lying above this upper limit cannot be realized inpractice.

The hot-rolled steel strip according to the invention must be at least1.5 mm thick, so that the Mn segregations desired according to theinvention can form in the structure after hot rolling. With smallerstrip thicknesses, the hot-rolled steel strip would experienceexcessively strong deformations during hot rolling, which in turn wouldresult in an undesired homogenization of the Mn distribution in thestructure of the hot-rolled steel strip. A steel strip with a thicknessof more than 10 mm cannot be used for the intended use. Therefore, themaximum strip thickness is limited to 10 mm.

The coiling temperature at which the hot-rolled steel strip, which formsthe steel substrate of the flat steel product according to theinvention, is coiled is at least 510° C. in order to secure theformation of Mn segregations during the cooling of the hot-rolled steelstrip in the coil. Higher coiling temperatures can promote this process,such that coiling temperatures of at least 530° C., in particular atleast 550° C., are particularly advantageous. At excessively low coilingtemperatures, an undesired homogeneous Mn distribution would result,with which the mechanical properties desired according to the inventionwould not be achieved. An excessively high coiling temperature wouldtrigger the risk of pronounced grain boundary oxidation. To preventthis, the coiling temperature is limited to 640° C., preferably 620° C.

After cooling in the coil, the hot-rolled steel strip can, if necessary,be pickled in a conventional manner, in order to remove scale present onthe steel strip or to prepare the surface of the steel strip for thework steps carried out below.

For the hot-dip coating, the hot-rolled steel strip is first heated toan annealing temperature at a heating rate of 0.5-100° C. per second ina pre-heating stage. The heating rate must lie within this window inorder to ensure sufficient conversion of the structure, in particularits complete recrystallization. For the same reason, an annealingtemperature of 750-950° C. and a holding time of 10-1000 seconds arerequired. At excessively low annealing temperatures or excessively shortholding times, the structure would not crystallize completely with theresult that, during the subsequent cooling, insufficient austenite wouldbe available to form the desired martensite proportion of the structure.An unrecrystallized steel substrate would also result in a pronouncedanisotropy of the mechanical properties of a flat steel productaccording to the invention.

Cooling from the annealing temperature to the zinc bath entrytemperature BET likewise takes place at a cooling rate of 0.5 to 100° C.per second. The bath entry temperature BET is at least equal to and atmost 20° C. higher than the melt bath temperature, in order to preventthe melt bath temperature from changing substantially by the entry ofthe hot-rolled steel strip.

Optionally, a further heat treatment (“galvannealing”) can follow thehot-dip coating, with which the hot-dip coated flat steel product isheated up to 550° C., in order to burn in the previously appliedcorrosion protection layer.

Either immediately after the exit from the zinc bath or after theadditional heat treatment, the flat steel product obtained is cooled toroom temperature at a cooling rate of 0.5-100° C./s.

The flat steel product thus produced can optionally be subjected to aconventional skin-pass rolling, in order to optimize its dimensionalaccuracy and surface properties. The degree of skin passing set here istypically at least 0.3% and at most 2.0%, wherein degrees of skinpassing of at least 0.5% have proven to be particularly practical. Adegree of skin passing of less than 0.3% leads to a lower surfaceroughness of the corrosion protection layer, which would have a negativeinfluence on the formability of the flat steel product. At a degree ofskin passing of more than 2.0%, the yield point Rp0.2 is increased andthe elongation at break A80 is reduced, so that an elongation at breakaccording to formula 1 could not be achieved.

Surprisingly, it has been found that a flat steel product that comprisesa steel substrate that is alloyed according to the invention and has astructure according to the invention achieves high elongation at breakvalues in the hot-rolled state, which are comparable with theelongations at break A80, which conventionally cold-rolled flat steelproducts of the type explained at the outset have (“DP-K steels”), whichhave similar strengths. Thus, in practice, elongation at break valuesA80 can regularly be achieved, for which the parameter B in formula (1)is at least in the range 31-51, preferably 36-46.

The combination of high strength and high elongation at break valuesresults from the proportion of 2-15% by area residual austenite presentin the steel substrate of a flat steel product according to theinvention, wherein residual austenite proportions of at least 5% by areaare regularly present in the structure of the steel substrate of a flatsteel product according to the invention and have a positive effect onthe mechanical properties of the flat steel product. The residualaustenite contents that can be established in the flat steel productaccording to the invention are thus significantly higher than with acold-rolled flat steel product with a comparable alloy.

According to the findings of the invention, the presence of largerresidual austenite proportions in the structure is a result of theinheritance of Mn segregations that are present in the steel substrate,hot-rolled according to the invention, of a flat steel product accordingto the invention and that are maintained via the annealing treatment,which the flat steel product passes through for its hot-dip coating. Itcould thus be shown that, in the manner according to the invention ofproducing a flat steel product according to the invention after coiling(sub-step A.6 of the method according to the invention) and beforehot-dip coating (work step B of the method according to the invention),the hot-rolled steel substrate has a highly anisotropic andinhomogeneous structure with a high pearlite content, which is presentin line form. Wavelength-dispersive X-ray microrange analyses (WDX) ofthe structure result in the fact that Mn in the pearlite lines issegregated and the Mn segregations are present in a highly anisotropicand inhomogeneous distribution after coiling and before hot-dip coating.

With the hot-dip coating taking place in a continuous pass, the steelsubstrate of a flat steel product according to the invention passesthrough an annealing (sub-step B.2 of the method according to theinvention) before entry into the melt bath, during which it is kept atthe annealing temperature over a period of time. Thereby, according tothe invention, the annealing temperature and the annealing duration arecoordinated with one another in such a manner that there is noredistribution of the Mn segregations. Therefore, in the case of thefinished hot-dip coated flat steel product according to the invention,despite the annealing treatment required for the preparation of the Zncorrosion protection coating, an anisotropic and inhomogeneous Mndistribution in the steel substrate is also present, which, as such, hasbeen “inherited” from the end structure present after the coiling of thehot-rolled steel substrate of the flat steel product.

Given that Mn contributes very strongly to the stability of theaustenite during the annealing in the intercritical region, both theconversion temperature and the residual austenite content after coolingare distributed in a more inhomogeneous manner in comparison tohot-rolled flat steel products that were coiled at lower temperatures indeviation from the specification of the invention. With a flat steelproduct produced according to the invention, the structural regions ofthe steel substrate in which there is a higher Mn concentrationtransform more easily and thus retain more austenite after cooling thanthe structural regions in which a lower Mn concentration is present.They convert at higher temperatures or not at all, whereby a higherproportion of the original ferrite is maintained there.

The inhomogeneity of the Mn distribution in the steel substrate of afully processed flat steel product according to the invention can bequantified by the total surface proportion of the structure of the steelsubstrate in which an Mn concentration (in % by mass) is present whichis more than 15% higher than the average value of the Mn concentrationsin the entire structure of the flat steel product. The sum of thesurface proportions of the structure of the steel substrate of a flatsteel product according to the invention which have an Mn concentrationthat is more than 15% higher than the average value of the Mnconcentration in the entire structure is referred to as “X.” In a flatsteel product according to the invention, X is at least 10%, inparticular at least 12%, advantageously at least 15% of the totalstructure. The surface proportions forming the sum X can be evaluatedusing a WDX measurement, wherein typically the Mn concentration isdetermined over a measurement surface of at least 200×200 μm with a stepsize of 0.5 μm.

The steel of the steel substrate of a flat steel product according tothe invention present in the course of the production according to theinvention as a hot-rolled steel strip is composed as follows:

Carbon (C) is present in the steel substrate of a flat steel productaccording to the invention in contents of 0.04-0.23% by mass. C is anessential element for the formation of martensite and austenite, whichare required in order to achieve the strength properties required by aflat steel product according to the invention. In order for this effectto occur to a sufficient extent, the steel according to the inventioncontains at least 0.04% by mass, wherein the desired effect is achievedparticularly reliably at C contents of at least 0.07% by mass. Anexcessively high C content would have a negative effect on the weldingbehavior of the flat steel product. In general, the weldability of asteel decreases with the level of its C content. In order to avoidnegative influences of the C content on its processability, the Ccontent of the steel according to the invention is therefore limited toa maximum of 0.23% by mass, in particular to a maximum of 0.20% by mass,wherein the negative effects of the presence of C can be particularlyreliably avoided at contents of at most 0.17% by mass.

Silicon (Si) is present in the steel substrate of a flat steel productaccording to the invention in contents of 0.04-0.54% by mass. Si isrequired to suppress the formation of pearlite in the structure duringannealing, which would have a negative effect on the mechanicalproperties of the end product. A minimum content of 0.04% by mass Si isrequired for this purpose. An excessively high Si content also preventsthe formation of pearlite during coiling and thus the segregation of Mnin the structure of the steel substrate. A significant segregation of Mnduring coiling is necessary to achieve a high sum X and the desiredmechanical properties. An excessively high Si content would likewiseimpair the surface quality of a flat steel product according to theinvention. For these reasons, the upper limit of the Si content islimited to 0.54% by mass.

Aluminum (Al) can optionally be added to the steel substrate of a flatsteel product according to the invention in contents of 0.01-1.5% bymass, in order to contribute to the suppression of the formation ofpearlite. Even if Al is used in the usual manner for deoxidation of themelt, a minimum Al content of 0.01% by mass results. However, anexcessively high Al content can have a negative effect on thecastability of the steel and worsen the coating behavior during thehot-dip coating. Such negative influences of the presence of Al in thesteel of the substrate of a flat steel product according to theinvention can thereby be avoided particularly reliably in that the Alcontent is limited to at most 1.0% by mass, in particular at most 0.5%by mass.

Manganese (Mn) is present in the steel substrate of a flat steel productaccording to the invention in contents of 1.4-2.9% by mass. Mn is amixed crystal element that contributes to the strength of the material.The presence of Mn in the steel of the substrate of a flat steel productaccording to the invention additionally stabilizes the austenite in thestructure of the substrate. The special feature of the alloy conceptaccording to the invention in combination with the production accordingto the invention of a flat steel product according to the inventionconsists in that a flat steel product according to the invention is anoptimal combination of high tensile strength and high elongation atbreak as a result of the segregation of Mn in the pearlite lines of thesteel substrate after coiling, which is also maintained if the flatsteel product has been annealed for the hot-dip coating and has passedthrough the hot-dip bath. In order for Mn to be enriched to a sufficientdegree in the pearlite lines by segregation, Mn contents of at least1.4% by mass are required, wherein it is favorable with regard to thereliability with which the positive influence of Mn on the properties ofa flat steel product according to the invention is established when theMn content is at least 1.5% by mass. However, an excessively high Mnconcentration would also have a negative effect on weldability.Therefore, the upper limit of the Mn content of the steel substrate of aflat steel product according to the invention is limited to 2.9% bymass, preferably 2.5% by mass, wherein the amount of Mn for theproperties of a flat steel product according to the invention can beutilized particularly effectively at Mn contents of up to 2.2% by mass.

Chromium (Cr) and molybdenum (Mo) can be added to the steel of the steelsubstrate of a flat steel product according to the invention as optionalelements for increasing strength. In addition, the presence of Cr and/orMo increases the formation of martensite with respect to pearlite duringthe cooling of the flat steel product from the intercritical region in acontinuous coating line. If these effects are to be utilized, contentsof Cr and Mo that in total amount to at least 0.02% by mass, inparticular at least 0.05% by mass, are required. In the case ofexcessively high Cr contents, however, the risk of pronounced grainboundary oxidation would be increased. An excessively high Mo content isalso to be avoided for reasons of cost. In order to be able toeffectively utilize the effects of Cr and Mo in the steel of the steelsubstrate of a flat steel product according to the invention, the upperlimit of the total content of Cr and Mo is therefore set to 1.4% bymass, preferably 1.0% by mass. Thereby, Cr and Mo do not necessarilyhave to occur in combination, but can also each be added alone to thesteel in contents of 0.02 to 1.4% by mass, in particular 0.05-1.0% bymass, as specified according to the invention, in order to achieve theeffects explained. However, particularly favorable effects result whenCr and Mo are present together, each in effective contents, as long asthe sum of such contents is within the limits according to theinvention.

At least one of the elements of titanium (Ti) and vanadium (V) ispresent as a required constituent in the steel of the steel substrate ofa flat steel product according to the invention in contents of0.005-0.15% by mass, wherein, here as well, it applies that an optimaleffect of such elements occurs when Ti and V are each present togetherin effective contents. Ti and V are micro-alloying elements that causethe formation of fine precipitates in the steel. Such precipitatesprevent the coarsening of the austenite grains at temperatures that arehigher than the Ar1 temperature of the steel, and in this manner lead tothe refinement of the structure. A finer structure favors thesegregation of Mn that is desired according to the invention during thecoiling carried out in the course of the production of a flat steelproduct according to the invention, because the distance over which Mndiffuses is reduced by the presence of Ti and/or V. Ti-containing andV-containing precipitates also contribute to the strength of a flatsteel product according to the invention by dispersion hardening. Toachieve these effects of Ti and V, Ti and/or V contents of at least0.005% by mass in total are required. At contents above 0.15% by mass,the presence of Ti and/or V no longer results in any particular increasewith regard to the properties desired according to the invention.Rather, Ti and V can be utilized particularly effectively if the sum oftheir contents is at most 0.1% by mass.

According to the invention, the content of niobium (Nb) is limited toless than 0.005% by mass, so that, if niobium is present at all, it isamong the impurities that are technically ineffective. Higher Nbcontents would lead to the formation of fine Nb precipitates, whichwould bring about susceptibility to crack formation during continuouscasting or in the case of the slab cooling or reheating. Therefore, theNb content is preferably limited to less than 0.003% by mass, inparticular less than 0.002% by mass.

Boron (B) can likewise optionally be added to the steel of the steelsubstrate of a flat steel product according to the invention in contentsof 0.0005-0.005% by mass, in order to prevent the formation of ferritefrom the intercritical region in the course of the cooling carried outduring the production of the flat steel product. In this manner, Bpromotes the formation of bainite, which leads to an increase instrength. For this purpose, a minimum content of 0.0005% by mass B isrequired, but excessively high B content can lead to undesiredembrittlement. Therefore, according to the invention, the upper limit ofthe B content, if B is added, is set to not more than 0.005% by mass, inparticular 0.002% by mass.

Phosphorus (P) is among the undesired, but technically generallyunavoidable impurities of the steel of the steel substrate of a flatsteel product according to the invention and should therefore be as lowas possible. P proves to be disadvantageous in particular with regard toweldability. In order to reliably avoid its unfavorable influence, the Pcontent according to the invention is limited to less than 0.02% bymass, preferably less than 0.01% by mass, in particular less than 0.005%by mass.

Sulfur (S) is also among the undesired, but technically generallyunavoidable impurities of the steel of the steel substrate of a flatsteel product according to the invention and should therefore be as lowas possible. At higher concentrations, S leads to the formation of MnSor (Mn, Fe)S, which would have a negative effect on the elongationbehavior of a flat steel product according to the invention. In order toavoid such unfavorable effects, the S content according to the inventionis limited to less than 0.005% by mass, preferably less than 0.002% bymass.

Nitrogen (N) also includes the undesired, but technically generallyunavoidable impurities of the steel of the steel substrate of a flatsteel product according to the invention and should therefore be as lowas possible. N forms, for example, nitrides with aluminum or titanium.In the case of higher N contents, this would lead to coarse precipitatesthat could be harmful to the formability of the flat steel product.Therefore, the N content is limited according to the invention to lessthan 0.01% by mass, preferably less than 0.005% by mass.

In conventional steel production, calcium (Ca) also enters the steelbecause it is added both for deoxidation and desulfurization and toimprove castability. An excessively high concentration of Ca can lead tothe formation of undesired inclusions, which have a negative effect onmechanics and rollability. Therefore, the upper limit of the Ca contentis limited to at most 0.005% by mass, preferably at most 0.002% by mass,

Copper (Cu), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zirconium(Zr), lanthanum (La) and/or cerium (Ce) are alloying elements that arealso among the impurities of the steel of the steel substrate of a flatsteel product according to the invention, the presence of which isundesirable per se. In order to reliably prevent influences of suchelements on the properties of a flat steel product according to theinvention, in the steel of the steel substrate of a flat steel productaccording to the invention, the Cu content is limited to not more than0.2% by mass, the Ni content is limited to not more than 0.1% by mass,the Sn content is limited to not more than 0.05% by mass, the As contentis limited to not more than 0.02% by mass, the Co content is limited tonot more than 0.02% by mass, the Zr content is limited to not more than0.0002% by mass, the La content is limited to not more than 0.0002% bymass, and the Ce content is limited to not more than 0.0002% by mass.

Oxygen (O) is also an undesirable impurity, since in the presence oflarger amounts of O, oxide deposits are formed, which have a negativeeffect both on the mechanical properties of the flat steel product andon the castability and rollability of the steel of its steel substrate.Therefore, the content of oxygen is limited to at most 0.005% by mass,preferably 0.002% by mass.

Hydrogen (H) is also among the undesirable impurities of the steel ofthe steel substrate of a flat steel product according to the invention.As the smallest atom, H is highly mobile on interstitial sites in thesteel and can lead to cracking in the core during cooling from hotrolling, in particular in ultrahigh-strength steels. Therefore, thecontent of H in the steel of the steel substrate of a flat steel productaccording to the invention is reduced to a maximum of 0.001% by mass,preferably a maximum of 0.0006% by mass, more preferably a maximum of0.0004% by mass, most preferably a maximum of 0.0002% by mass.

No particular requirements are imposed on the composition of thecorrosion protection coating and the associated melt bath through whichthe flat steel product passes during its hot-dip coating. Thus, thecorrosion protection coating of a flat steel product according to theinvention consists of zinc (Zn) in its main proportion and can otherwisebe composed in a conventional manner.

Accordingly, in addition to Zn and unavoidable impurities, the corrosionprotection layer can contain up to 20% by mass Fe, up to 5% by mass Mgand up to 10% by mass Al. Typically, if they are each present, at least5% by mass Fe, at least 1% by mass Mg and/or at least 1% by mass Al isprovided, in order to achieve optimal usage properties of corrosionprotection.

The invention is explained in more detail below with reference toexemplary embodiments.

To test the invention, steels A-1 were melted and cast into slabs, thecomposition of which is specified in Table 1. Contents of an alloyingelement that are so small that they are “0” in the technical sense,i.e., are so small that they have no influence on the properties of thesteel, are referred to in Table 1 by the entry “-”.

The slabs were heated through in a preheating furnace in which apreheating temperature VT prevailed.

Subsequently, the preheated slabs were hot rolled in a conventionalmanner to form hot-rolled steel strips W1-W35, wherein the hot rollingwas ended at an end rolling temperature ET.

The hot-rolled steel strips W1-W35 obtained in this manner were coiledin a likewise conventional manner starting from a coiling temperature HTin a likewise conventional manner to each form a coil. If necessary,they were cooled to the coiling temperature HT in a conventional mannerfor this purpose before coiling.

To demonstrate the effect of the invention, in the production of thehot-rolled steel strips W1-W35, which consisted of one of the steels A-1in each case, one of the combinations I-VIII specified in Table 2 ofpre-heating furnace temperature VT, hot-rolling end temperature ET andcoiling temperature HT was selected in each case. The preheating furnacetemperatures VT, hot-rolling end temperatures ET and coilingtemperatures HT belonging to each of the combinations I-VIII arespecified in Table 2. Thereby, those preheating furnace temperatures VT,hot-rolling end temperatures ET and coiling temperatures HT which ineach case do not correspond to the specifications of the invention areemphasized by underlining.

After cooling in the coil, the hot-rolled steel strips W1-W35 werecoated with a Zn-based corrosion protection layer by hot-dip coating.For this purpose, they were subjected in each case to one of sixvariants a-f of an annealing treatment and a melt application, in whichthey were heated in a pre-heating stage with a heating rate HR to anannealing temperature GT, at which they were subsequently held over anannealing period of 40 s to 100 s in each case. Subsequently, thehot-rolled steel strips W1-W35 were cooled with a cooling rate KR1 to abath entry temperature BET, which was in each case equal to the bathtemperature of the melt bath, through which the hot strips were passedafter the respective annealing treatment a-f. The melt bath consisted ofat least 99% by mass Zn. The now complete flat steel products emergingfrom the melt bath and produced on the basis of the hot-rolled steelstrips W1-W35 were subsequently cooled to room temperature at a coolingrate KR2. The parameters of heating rate HR, annealing temperature GT,cooling rate KR1, bath entry temperature BET and cooling rate KR2belonging to the variants a-f of the annealing treatment and the meltapplication are recorded in Table 3.

The mechanical properties and structural constituents of the flat steelproducts obtained in the manner explained above were determined. Theresults of these investigations of yield point Rp0.2, tensile strengthRm, elongation at break A80, parameter “B” from formula (1), ferriteproportion F of the structure, martensite proportion M of the structure,austenite proportion A of the structure, proportion SO of the otherconstituents of the structure and sum X of the surface proportions ofthe structure of the steel substrate, in which there is a Mnconcentration that is more than 15% above the average value of the Mnconcentration in the structure are summarized in Table 4, where, for theflat steel products produced on the basis of the hot-rolled steel stripsW1-W35, it is also specified which of the steels A-1 the steel substrateof the respective flat steel product consisted of, and which of thecombinations I-VIII of hot strip production (“WEZ” column) and which ofthe variants a-f of the annealing treatment and the melt application therespective steel substrate passed through (“GS” column).

The flat steel products produced from the hot-rolled steel strips W1,W3, W6, W7, W8 and W27 were not produced in accordance with theinvention:

With the flat steel product produced from the hot-rolled steel strip W1,the slab was heated with an excessively low preheating temperature VT,so that the slab was not fully annealed. As a result, the alloyingelements and the production processes did not affect the mechanicalproperties.

The hot-rolled steel strip W3 contains too little Mn, so that Mn in thepearlite lines of the hot strip structure did not segregate tosufficient degrees. This resulted in a lower residual austenite contentand therefore to a relatively low elongation at break A80 of the flatsteel product produced from the hot-rolled steel strip W3. As a result,parameter B was below 31.

In the production of the hot-rolled steel strips W6, W7 and W8,excessively low coiling temperatures were set. This led to a similareffect on the Mn segregations and therefore to insufficient mechanicalproperties, as in the flat steel product produced from the hot-rolledsteel strip W3.

During the annealing treatment of the hot-rolled steel strip W27, anexcessively low GT was set, so that the structure was not completelyrecrystallized. This resulted in a low austenite content in thestructure of the steel substrate of the flat steel product obtained andtherefore to a low elongation at break A80.

TABLE 1 Content information in % by mass Remainder of iron andunavoidable impurities Steel C Si Al Mn Cr + Mo Ti + V Nb B P S N A0.043 0.052 0.025 1.41 0.02 0.008 0.003 — 0.004 0.004 0.003 B 0.0510.061 0.023 1.23 0.139 0.011 0.004 — 0.004 0.003 0.004 C 0.071 0.1070.021 1.51 0.043 0.038 0.004 0.0018 0.005 0.002 0.004 D 0.087 0.0420.172 1.92 0.350 0.057 0.002 — 0.007 0.003 0.003 E 0.142 0.053 0.1851.62 0.396 0.031 0.002 0.0012 0.004 0.004 0.003 F 0.150 0.487 0.016 2.000.329 0.051 0.003 — 0.008 0.004 0.003 G 0.168 0.193 0.980 2.15 0.6940.091 0.004 — 0.009 0.002 0.004 H 0.192 0.048 1.46 2.41 0.986 0.1050.004 — 0.012 0.003 0.007 I 0.222 0.54 0.093 2.76 1.367 0.143 0.0020.0046 0.018 0.004 0.009

TABLE 2 Temperature information in ° C., underlined values are notaccording to the invention Combination VT ET HT According to theinvention? I 1100 850 550 NO *) II 1150 890 500 NO *) III 1200 910 520YES IV 1250 930 540 YES V 1150 890 560 YES VI 1200 910 590 YES VII 1250930 610 YES VIII 1150 890 630 YES *) Parameters not according to theinvention are underlined

TABLE 3 Underlined values are not according to the invention. HR GT KR1BT KR2 Variant [° C./sec] [° C.] [° C./sec] [° C.] [° C./sec] a 7 725 4455 6 b 12 750 5 465 8 c 23 775 7 460 11 d 41 800 10 470 13 e 60 825 14465 16 f 73 850 18 475 19

TABLE 4 Rp0.2 Rm A80 F M A SO X Strip Steel WEZ GS [MPa] [%] B [% bymass] [%] W1 A I e 368 523 16.5 30.6 70.0 10.0 1.5 18.5 13.7 W2 A III e311 503 28.9 42.5 80.0 15.0 3.5 1.5 10.5 W3 B III e 286 483 17.3 30.485.0 12.0 1.0 2.0 12.1 W4 C III c 340 569 25.7 41.1 77.0 16.0 4.5 2.511.6 W5 C III f 348 675 25.9 44.1 73.0 20.0 5.0 2.0 10.2 W6 D II b 432754 9.6 30.0 65.0 32.0 1.5 1.5  8.4 W7 D II d 364 687 10.9 29.5 64.033.0 1.0 2.0  9.9 W8 D II f 363 708 11.3 30.4 62.0 35.0 1.5 1.5  8.9 W9D V b 498 783 13.5 34.7 67.0 27.0 3.0 3.0 13.8 W10 D V d 475 580 23.138.8 76.0 20.0 2.5 1.5 14.2 W11 D V f 351 700 20.3 39.2 71.0 25.0 2.51.5 12.9 W12 D VIII b 385 681 24.6 43.0 70.0 22.0 6.0 2.0 17.8 W13 DVIII d 352 661 26.7 44.6 68.0 24.0 6.0 2.0 18.5 W14 D VIII f 343 67326.6 44.8 71.0 21.0 5.0 3.0 19.3 W15 E III b 615 1030  14.5 42.3 58.038.0 2.0 2.0 13.2 W16 E III d 581 995 15.3 42.2 58.0 37.0 2.5 2.5 12.4W17 E III f 870 1190  9.0 41.2 56.0 40.0 2.0 2.0 12.9 W18 E VI b 416 75617.3 37.7 67.0 28.0 3.0 2.0 17.1 W19 E VI d 423 787 21.2 42.5 64.0 27.06.5 2.5 16.8 W20 E VI f 503 839 22.1 44.8 63.0 27.0 7.5 2.5 17.7 W21 FIV b 509 947 13.2 38.8 62.0 30.0 6.0 2.0 12.2 W22 F IV d 431 875 15.939.5 65.0 26.0 7.5 1.5 12.4 W23 F IV f 423 858 15.6 38.8 65.0 26.0 7.02.0 13.5 W24 F VII b 510 921 16.7 41.6 60.0 28.0 10.0  2.0 16.8 W25 FVII d 407 813 22.8 44.8 66.0 21.0 10.0  3.0 17.8 W26 F VII f 420 80123.2 44.8 68.0 21.0 8.5 2.5 18.2 W27 G IV a 382 779 9.5 30.6 65.0 20.00.5 14.5 13.5 W28 G IV c 489 892 19.6 43.7 65.0 23.0 9.5 2.5 12.2 W29 GIV e 445 822 22.9 45.1 67.0 21.0 10.5  1.5 12.7 W30 H V b 475 875 14.438.0 68.0 25.0 4.5 2.5 15.1 W31 H V d 519 875 20.8 44.4 63.0 24.0 11.0 2.0 15.9 W32 H V f 613 951 17.7 43.4 63.0 23.0 12.0  2.0 15.6 W33 I VI b600 1030  14.8 42.6 55.0 29.0 13.0  3.0 16.2 W34 I VI d 569 981 18.244.7 55.0 27.0 14.5  3.5 16.3 W35 I VI f 851 1190  7.3 39.5 49.0 35.014.0  2.0 17.5 Underlined values are not according to the invention.

1. A hot-rolled flat steel product, which comprises a steel substrate at least 1.5 mm thick, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of elements selected from the group consisting of Al, Cr, Mo, and B, wherein: AI: 0.01-1.5%, the sum of the contents %Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and B: 0.0005-0.005%, and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb, wherein the hot-rolled flat steel product has a structure which consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production, has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 determined by the following formula (1): A80[%]=B−Rm/37 with 31≤B≤51, and a corrosion protection layer based on zinc is applied to at least one surface of the steel substrate by hot-dip coating.
 2. The flat steel product according to claim 1, wherein the structure of the steel substrate contains at least 5% by area residual austenite.
 3. The flat steel product according to claim 1, wherein, for the parameter B of formula (1), the following applies: 36≤B≤46.
 4. The flat steel product according to claim 1, wherein the sum X of surface proportions of the structure of the steel substrate in which there is a Mn concentration that is more than 15% above the average value of the Mn concentration in the structure is at least 10% of the total structure.
 5. The flat steel product according to claim 4, wherein the sum X is at least 12%.
 6. The flat steel product according to claim 5, wherein the sum X is at least 15%.
 7. The flat steel product according to claim 1, wherein the corrosion protection layer contains at least 75% by mass Zn.
 8. A method for the production of a flat steel product according to claim 1 comprising: A) producing a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps: A.1) melting a steel melt, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of the elements selected from the group consisting of Al, Cr, Mo, and B, wherein AI: 0.01-1.5%, the sum of the contents of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and B: 0.0005-0.005%, and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb; A.2) casting the steel melt to form a preliminary product, which is a slab or thin slab; A.3) preheating the preliminary product at a preheating temperature of at least 1150° C. and at most 1350° C.; A.4) hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm; A.5) cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.; and A.6) coiling the hot-rolled steel strip cooled to the coiling temperature; and B) coating the steel substrate, which is present in the form of a hot-rolled steel strip, with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously: B.1) optional pickling of the hot-rolled steel strip; B.2) heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s; B.3) cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.) applies, wherein BT is the temperature of the zinc melt bath and is 450-480° C.; B.4) passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass AI, and a remainder of Zn and unavoidable impurities to obtain a flat steel product; B.5) cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s; and B.6) optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%.
 9. The method according to claim 8, wherein the coiling temperature is at least 530° C.
 10. The method according to claim 9, wherein the coiling temperature is at least 550° C.
 11. The method according to claim 8, wherein the coiling temperature is at most 620° C. 